1. FIELD OF THE INVENTION
The present invention relates to a process for purifying crude anthraquinone (hereinafter referred to as "AQ") obtained by the oxidation, by molecular oxygen in the presence of a basic compound, of an adduct obtained by the Diels-Alder (hereinafter referred to as "DA") reaction of butadiene (hereinafter referred to as "BD") with 1,4-naphthoquinone (hereinafter referred to as "NQ") obtained by a catalytic vapor phase oxidation reaction of naphthalene.
2. DESCRIPTION OF THE PRIOR ART
AQ is an extremely important industrial material as an intermediate for dyestuffs. Recently, it has been found that AQ is useful also as a digesting assistant for pulps, and an attention has been drawn to its functions.
As a digesting assistant for pulps, AQ is desired to be inexpensive, available in fine particles and highly dispersible when added to water or white liquor.
As a popular method for the production of AQ, a so-called DA process is known in which an adduct obtained by the DA reaction of NQ with BD is oxidized in an aqueous medium in the presence of a basic compound. Crude AQ obtainable by this process is blackish brown and contains polycondensation products of NQ and by-products of the DA reaction, and it usually has a purity of not higher than 98%.
As an industrial method for the purification of the crude AQ, distillation is commonly used.
However, the melting point of AQ is extremely high at a level of 286.degree. C. Accordingly, it is likely that polycondensation of the impurities and AQ proceeds during the melting or storage of the crude AQ or during the distillation, and the yield of refined AQ by the distillation will thereby be low. There are additional problems involved in the withdrawal of the residue, the recovery of AQ from the residue and the necessity of the treatment of the residue.